HPs x Wind drag

[RACarvalho]

Ok, I have seeing several posts about what TV would be best to tow a hybrid but usually the conversation is around weight carrying capacity, etc.
But what about engine Horsepower?

Since I could not find one about HP's I did the calculation for how much HP's are needed to win over wind resistance at 63 miles per hour towing a trailer with an 11'x8' frontal area.
@ 63 miles an hour you need 151 HPs only to win over wind resistance.
If you add 10 miles per hour of contrary wind, you need 235 horses....
And that only to win over the wind resistance !!!

There are some leeway on the calculation for the drag coefficient used is the one of a plate and does not consider the aerodynamics of the vehicle in front of the trailer, which should reduce that a little bit...

Does this match your experience?

[raspivey]

It's just not practical to consider horsepower alone. Gear ratio plays as much, if not more, of a role here. Payload capacity is right up there with that. I've got a 630 HP Corvette...theoretically, I can pull 10,000 lbs in a straight line..won't happen though.

I see what you're doing and it's interesting, but the reason you won't see much discussion about towing and HP is that there are too many other factors that influence it. Torque vs HP vs wind drag vs weight, on the other hand, could be an interesting graph.

[RACarvalho]

Originally Posted by raspivey

It's just not practical to consider horsepower alone. Gear ratio plays as much, if not more, of a role here. Payload capacity is right up there with that. I've got a 630 HP Corvette...theoretically, I can pull 10,000 lbs in a straight line..won't happen though.

I see what you're doing and it's interesting, but the reason you won't see much discussion about towing and HP is that there are too many other factors that influence it. Torque vs HP vs wind drag vs weight, on the other hand, could be an interesting graph.

Yes, torque and gear ratio helps when accelerating and going up hill but to keep speed you need HPs...
My curiosity is this:
Per my calculations, if you are traveling at 65mph and have a headwind of 10mph you need 254HPs... That is pretty close to ~ 280Hps that some trucks have so I'd like to know if people have faced this kind of situation... ie.: how close the numbers are from reality.

[raspivey]

I've towed with about 270 HP into a 25 mph headwind at around 60 mph. It didn't like it but, if I had to guess, I'd say that I'd probably have run out of power at around 80 mph. I was at about 60% throttle at the time.

[rsdata]

the only thing I can add is that my 5.2 liter Dodge magnum engine peaks at 240Hp at 4400 RPM... I typically run at 3100 RPM about 62-65 MPH and the Hp curve goes down to 210 Hp at that RPM... I have 4.10 gears though that makes up for that low of horsepower. I also have 195K miles on the engine so it is probably not quite up to those numbers.

Per my calculations, if you are traveling at 65mph and have a headwind of 10mph you need 254HPs... That is pretty close to ~ 280Hps that some trucks have

not sure how your calculations jive with my reality

[schrederman]

In your calculations you probably noticed that air drag goes up with the square of speed. So, if you know what it takes to tow at 30 mph, you know that power required doesn't just double at 60 mph, but is 4 times as great. Increase the speed to 90 mph instead of 60, and it goes up by 9 times.

[RACarvalho]

Originally Posted by rsdata

the only thing I can add is that my 5.2 liter Dodge magnum engine peaks at 240Hp at 4400 RPM... I typically run at 3100 RPM about 62-65 MPH and the Hp curve goes down to 210 Hp at that RPM... I have 4.10 gears though that makes up for that low of horsepower. I also have 195K miles on the engine so it is probably not quite up to those numbers.



not sure how your calculations jive with my reality

yes, based on Raspivey and your feedback C (drag coefficient) in my formula needs to be reduced.
As I stated before I used the "flat sign" coefficient (1.075)...
What is your speed and what % of throttle you are when towing at 3100 rpm?

Thank you,

[RACarvalho]

Originally Posted by schrederman

In your calculations you probably noticed that air drag goes up with the square of speed. So, if you know what it takes to tow at 30 mph, you know that power required doesn't just double at 60 mph, but is 4 times as great. Increase the speed to 90 mph instead of 60, and it goes up by 9 times.

Yes, the formula I'm using is this:
F = 0.5*C*p*A*V²

[lbrjet]

No trailer has an eleven foot high frontal area. Measure the actual box, not from the ground. Considering the truck is going to displace a lot of the air out of the way first, this would also change the calculations. The top of my F250 is almost 7 feet high and it has a large, tall front end as well. According to the torque curve for my truck I am running at about 100 HP at 65 mph and 1800 RPM's in overdrive. I can do this all day long on a non windy day. Add wind and I am at 160 HP at 60 mph and 2500 RPM's. Once heading into a 30-40 mph head wind in Texas (and slightly uphill the whole way) I was at 260 HP at 60 mph and 3700 RPM's. My truck and trailer weights together total 98% of my GCWR.


2010 F250 5.4L 3.73
2011 Flagstaff 831FKBSS
Equal-i-zer 4pt 12K

[ebcobra93]

I pull a Roo 183 with a 2014 ford explorer sport (rated 365 HP, haven't looked at the torque curve). I'm at around 98-99% of GCWR when fully loaded. I've never felt under powered, but my mpg really suffers when driving over 65mph (<8 mpg). I'm sure alot of that is due to wind resistance.
I've thought about getting a wind deflector for my roof, but nothing I've read has convinced me that they make much of a difference.

[rsdata]

What is your speed and what % of throttle you are when towing at 3100 rpm?

I typically run at 3100 RPM about 62-65 MPH... I would guess I am at 70-80% of throttle at no head wind

[Swift 2]

Very interesting. Has any one out there researched any thing that really does reduce wind drag of our C?

[RACarvalho]

Originally Posted by Swift 2

Very interesting. Has any one out there researched any thing that really does reduce wind drag of our C?

Campingworld sells some wind deflectors which theoretically would improve aerodynamics.
Some of those claim 10-15% improvement in fuel consumption...
The only real test I saw on this subject was performed by CamAm and seems to indicate that the frontal slope of the trailer makes all the difference... but unfortunatelly Hybrids have to have a more vertical front because of the bed door...

[ChateauV10]

The Ford towing guide details the assumption they make on trailer front surface area. In fact, they have an entire page on the topic:
http://www.fleet.ford.com/resources/...Tgde_Sep30.pdf


And this note on an SAE standard they use:
Note: Mustang, Fusion, MKZ, Escape, MKC, Edge, Lincoln MKX, Flex, MKT, Transit Connect, Transit, F-150 and F-450 Super Duty pickup calculated with SAE J2807 method.


Cheers

[RACarvalho]

Originally Posted by lbrjet

No trailer has an eleven foot high frontal area. Measure the actual box, not from the ground. Considering the truck is going to displace a lot of the air out of the way first, this would also change the calculations. The top of my F250 is almost 7 feet high and it has a large, tall front end as well. According to the torque curve for my truck I am running at about 100 HP at 65 mph and 1800 RPM's in overdrive. I can do this all day long on a non windy day. Add wind and I am at 160 HP at 60 mph and 2500 RPM's. Once heading into a 30-40 mph head wind in Texas (and slightly uphill the whole way) I was at 260 HP at 60 mph and 3700 RPM's. My truck and trailer weights together total 98% of my GCWR.


2010 F250 5.4L 3.73
2011 Flagstaff 831FKBSS
Equal-i-zer 4pt 12K

Good catch on the trailer height, I'll correct that.
On the other hand, you are not at the stated power at a given RPM if you are not full throttle...

[DaGrinch]

Originally Posted by RACarvalho

Campingworld sells some wind deflectors which theoretically would improve aerodynamics.
Some of those claim 10-15% improvement in fuel consumption...
The only real test I saw on this subject was performed by CamAm and seems to indicate that the frontal slope of the trailer makes all the difference... but unfortunatelly Hybrids have to have a more vertical front because of the bed door...

Made my own deflector several years ago for the top of my '04 Jeep GC towing a 23SS. Used it for my last trip to Myrtle Beach back in '09..... didn't help much in the mountains of Western MD, but once I got in the flats of I-95 I went from about 12-13 mpg to 14-15. I figured in a round trip, I saved about one tank of gas. To me, the deflector was worth it.

[RACarvalho]

Originally Posted by DaGrinch

Made my own deflector several years ago for the top of my '04 Jeep GC towing a 23SS. Used it for my last trip to Myrtle Beach back in '09..... didn't help much in the mountains of Western MD, but once I got in the flats of I-95 I went from about 12-13 mpg to 14-15. I figured in a round trip, I saved about one tank of gas. To me, the deflector was worth it.

Tks for sharing.
If I remember CamAm test right, the longer trailer with a less vertical front had a 12% better fuel comsumprion than a small trailer with a flat wall... Looks like consistent with your experience.
I'm considering to do one for myself for I think the ones on the market are too low.
What material did you use for yours? Did you felt the deflector pushing the Jeep down further?

Tks,

[lbrjet]

HP is simply a function of torque and RPM. Whether or not you are at full throttle has nothing to do it. HP = (tq * RPM) / 5252. I can approximate my torque from the torque curve at a particular RPM, then calculate the HP.


2010 F250 5.4L 3.73
2011 Flagstaff 831FKBSS
Equal-i-zer 4pt 12K

[rovinglion]

For those to whom vectors remain a mystery, I will try to simplify. In general the power rquired to go down the road is dependent on two things. (1) the drag of the combined vehicle and (2) the component of the combined weight of the TV and TT resolved in the direction of travel. Simply put, on a level surface, the power required is a function of the drag, coefficient, frontal area, and the square of the speed. A 10% increase in speed will result in a 21% increase in the power required. . Whereas a 10% increase in frontal area will only result in a 10% increase in power required. If you throw in an incline, such as a grade, then you have to add in the power required to move the weight up the grade. Precise numbers are difficult to calculate because they depend on so many variables. Ultimately the power required must be converted to the force applied by the wheels on the pavement. So simply put, bigger TV's require more power. The more hills and valleys in your travel plans, the more power you will need to maintain your speed. Sounds logical right?

[Iggy]

Originally Posted by RACarvalho

Yes, the formula I'm using is this:
F = 0.5*C*p*A*V²

You are way over thinking what you can tow in relation to wind drag.
You have a 2006 BMW X5 which has curb weight of 4,652 lbs and a Gross Vehicle weight of 6,008 lbs.
With a full tank of fuel and cargo you don't have much left.
Wind coefficient/drag has fractional effect on your pulling torque.

Maybe I just don't see your logic on why you would want to calculate just a minimal effect on towing. Enlighten me.